Different liquid operating means, as for example fuel, are required for the operation of motor vehicles, which have to be stored in the motor vehicle. In particular at diesel motor vehicles the so-called SCR-procedure (selective catalytic reduction) can be used for the pollutant emission reduction, which requires an aqueous urea-water solution (HWL) as further operating means. The urea-water solution is required as reducing agent, which is injected into the exhaust gas pipe demand-oriented and which enables together with a SCR-catalytic converter the reaction of nitric oxides in the exhaust gas to harmless water and nitrogen.
The aqueous urea-water solution is kept and stored in a so-called HWL-tank. However the freezing point of a usual 32.5% urea-water solution is generally at −11° C., so that the urea-water solution freezes at a corresponding weather. Therefore it is necessary to provide a tank heater in the HWL-tank.
A usual tank heater consists of electric heating elements, so that an electric current can be converted into heat. Therefore two ohmic heating elements can be for example provided, which are switched in series. Those elements can be combined with positive temperature coefficient thermistors, so-called PTC-elements. Positive temperature coefficient thermistor comprise current conducting materials, which conduct the current better at lower temperatures than at higher temperatures. The electric resistance increases with an increasing temperature, so that the created thermal output sinks with an increasing temperature and a “self-regulations” of the heating element takes place. A qualifying HWL-tank heater can for example be built by two parallel interconnected PTC-elements, which are arranged between two ohmic heating elements that are switched in series.
A standard exhaust gas after-treatment requires a liquid urea-water solution, so that the function of the tank heater that is built into the HWL-tank has to be maintained. Usually an electric diagnosis or functionality checking of such an electric heating device takes place by controlling the heating device by a familiar power switch, which impinges the heating device with a certain load current. The power switch reports a current mirror (sens-current) of the load current back to a control unit. With the aid of the sens-current the functionality of the heating device can be checked. This electric diagnosis of heating device is however relatively inaccurate and can cause misdiagnoses. In particular at a sinking sens-current for example the type of the disorder, for example the power break of a PTC-element, cannot be clearly recognized. The reasons for that are usually based amongst other in the parallel switching or the switch connection of the PTCD-elements. Furthermore the individual PTC-elements have high tolerances in the range of approximately 30%. Also the sens-current, which is reported by the power switch, provides in particular at low current mirrors high tolerances and inaccuracies. In addition to that there are different environmental conditions of the fluid that is present in the tank, for example different fluid levels and pressures as well as the dynamic behavior of the PTC-elements at different tank temperatures.
For diagnosing or checking the functionality of an electric heating element in a fuel tank the German application DE 102 34 561 A1 suggests to use the characteristic current course curve of a PTC-element in order to be able to signalize a defect by detecting the strength of the heating current that flows through the PTC-element during a certain period of time compared to a default threshold value. However this procedure does not provide a satisfying solution if more than one heating element, thus in particular a heating element connection has to be checked, because in particular at parallel interconnected PTC-heating elements no clear statements can be made about the type and location of the disorder. Furthermore the current flow in the heating device underlies significant deviations, so that wrong statements can be easily made at this procedure.
The invention has therefore the task to provide a procedure for a reliable functionality checking of an electric heating device in order to be able to ensure that for example the required heating power can be produced at a HWL-tank. The procedure according to the invention shall in particular diagnose whether individual or several heating elements are disabled in its performance at parallel interconnected PTC-elements and for example have dropped or short-circuited.